This invention relates to a vortex flowmeter and, more particularly, to a simple and accurate vortex flowmeter unit for measuring air flow for a vehicle's engine.
A vortex flowmeter to which the principle is adapted that number of vortexes produced per unit of time by a vortex generator is proportional to the flow's velocity is well known as a typical flowmeter adaptable to various kinds of fluids because of its simple design and the possibility of having a vortex generator so shaped as to maintain a constant Strouhal number over a wide range of Reynold's numbers. On the other hand, since engines for vehicles are required to be low in polluting and high in efficiency, fuel should be mixed with air at an optimum fuel-to-air ratio and, in this respect, an accurate air-flow measurement is highly required. Many kinds of flowmeters, e.g. thermal-type flowmeters and so on are adopted for this purpose. Since vortex flowmeters are capable of accurately measuring the air's flow, varying within a wide range corresponding to the working conditions of engines from the idling position to operating at high speeds, they have recently been increasingly applied to fuel systems for engines, especially to high performance engines.
A conventional vortex flowmeter for a vehicle's engine comprises a flowmeter body wherein a vortex generator and an ultrasonic transmitting-receiving element for detecting a Karman vortex street, produced by the vortex generator, are unitarily included, a flow-rectifying device for the rectification of the air's flow to be introduced into the flowmeter's body and a duct for feeding measured air into a carburetor. The flow-rectifying device, the vortex generator body and the duct are separately made and assembled into one system for actual use. The duct is connected to the flowmeter's body by means of a specially fitted plate which has an opening for passing therethrough a flow of air to the flowmeter's body, a by-pass pipe forming a by-pass flow passage and an adjusting port for maintaining a constant ratio of the bypass flow of air to the main flow of air in the flowmeter's main body.
The above-mentioned conventional vortex flowmeter unit is characterized in that its measuring range can be practically widened by providing the flowmeter with a by-pass line and a ratio of the bypass flow to the main body's flow which can be corrected to a constant value through the adjusting port provided. However, since the main body and the by-pass line differ from each other in passage shape, the drag coefficient cannot always be constant over all required ranges of the flow's measurement and therefore an accurate compensation of the flow's ratio through the adjusting port is effective only for a certain limited flow range and compensation for the other flow range is made with accuracy to a specific allowance. In the conventional vortex flowmeter it has not been possible to obtain an increase in accuracy of the flow's measurement over the full required range of measurement. A separate construction of the main body and the fitting plate requires a separate molding in respective metal molds but also requires their additional assembly. The above-mentioned drawback of the conventional vortex flowmeter could not be avoided because the by-pass pipe was integrally molded with the fitting plate.